On SMP-capable kernels (e.g. generic distro kernel) the cs5535-clockevt
driver loads but is not actually used.

Setting cpumask to cpu_all_mask works for UP-only kernels, but if compiled
for SMP - though still running on the same UP hardware -
kernel/time/tick-common.c:tick_check_new_device() reads this as
"non-cpu-local" and silently ignores the device.

If we leave cpumask unset clockevents_register_device() will initialize it
and the cs5535-clockevt driver will be used no matter how the kernel was
compiled. Should anyone ever manage to stick a CS553x in an SMP system
(is this even possible?) then a warning will be printed. This is fine as
the cs5535-clockevt driver was never written/tested for SMP.

If bisecting led you here this patch may have exposed a pre-existing MFGPT
problem. Configure for UP-only and re-check.